The desirability of improving the efficiency of combustion in vehicle engines has long been recognized. For instance, Lyons and McKone in U.S. Pat. No. 2,086,775, and again in U.S. Pat. No. 2,151,432, disclose a method for improving combustion efficiency in an internal combustion engine by adding to the fuel what is described as "relatively minute quantities" of catalytic organometallic compounds. The Lyons and McKone patents, though, are directed solely to internal combustion engines and do not address the problem of NO.sub.x emissions from diesel engines.
In a unique application of catalytic technology described in International Publication No. WO 86/03492 and U.S. Pat. No. 4,892,562, Bowers and Sprague teach the preparation of diesel fuels containing fuel soluble platinum group metal compounds at levels of from 0.01 to 1.0 parts per million. The Bowers and Sprague results were corroborated and refined by the work of Kelso, Epperly, and Hart, described in "Effects of Platinum Fuel Additive on the Emissions and Efficiency of Diesel Engines," Society of Automotive Engineers (SAE) Paper No. 901 492, August 1990. Although the use of platinum group metal additives is effective, further nitrogen oxides reductions are still believed possible.
Moreover, in "Assessment of Diesel Particulate Control--Direct and Catalytic Oxidation," SAE Paper No. 81 0112, 1981, Murphy, Hillenbrand, Trayser, and Wasser have reported that the addition of catalyst metal to diesel fuel can improve the operation of a diesel trap. Among the catalysts disclosed is a platinum compound, albeit one containing chlorine, which is known to reduce catalyst effectiveness. In addition, the regeneration of a diesel trap by the use of a metallic additive which can include copper, nickel, cobalt, and, especially, iron, is discussed by M uller, Wiedemann, Preuss and Sch aidlich in "Diesel Particulate Filter System with Additive Supported Regeneration," ATZ Automobiltechnische Zeitschift 91 (1989).
Other researchers have considered the use of water-in-oil emulsions for improving combustion efficiency in diesel engines. For instance, DenHerder, in U.S. Pat. No. 4,696,638, discusses such emulsions and indicates that the positive effects therefrom include "cleaner exhaust." Although the disclosure of DenHerder refers to emulsions containing up to about 40% water, DenHerder is primarily directed to emulsions having only up to about 10% water in the form of droplets having a diameter of about 1 to about 10 microns.
Furthermore, in "Diesel Engine NO.sub.x Control: Selective Catalytic Reduction and Methanol Emission," EPRI/EPA Joint Symposium on Stationary NO.sub.x Control, New Orleans, La., March, 1987, Wasset and Perry have reported that NO.sub.x reductions of up to 80%, which are the levels desired for effective emission control, can be achieved in diesel engines using water and oil emulsions. They found, though, that emulsions of at least 60% water-in-oil are necessary to achieve such reductions. Unfortunately, such high water ratios can lead to increased emissions of carbon monoxide (CO) and unburned hydrocarbons. In addition, such high water levels can also create problems in emulsion stability and create corrosion and storage volume concerns.
Accordingly, a process and composition which is effective at substantially reducing the nitrogen oxides emissions from a vehicular diesel engine without the drawbacks of the prior art is extremely desirable.